This invention relates to a receiving arrangement for receiving a transmission radio signal through a transmission radio path and, in particular, to a receiving arrangement for use in a satellite communication system.
In a satellite communication system, communication is carried out between a central station and one or more peripheral or local stations via a transmission radio path which comprises a communication satellite in a frequency division fashion. The central station and the local stations are herein called communication stations. Each of the communication stations comprises a transmitting arrangement and a receiving arrangement. The communication satellite comprises usually a plurality of transponders as known in the art. Each of the transponders may be either an active one or a backup one. All of the transponders may be active ones.
In the satellite communication system, each communication station is preliminarily assigned with a transmission carrier frequency for use in the transmitting arrangement and a reception carrier frequency for use in the receiving arrangement. The transmission and the reception carrier frequencies are determined in relation to a specific transponder which is the active one at the time being.
The transmitting arrangement transmits via the specific transponder a transmission radio signal of the transmission carrier frequency assigned thereto. The specific transponder repeats the transmission radio signal to produce a repeated radio signal of a repeated carrier frequency which is shifted downwardly of the transmission carrier frequency by a predetermined frequency difference. The receiving arrangement receives the repeated radio signal by using a reception carrier signal of the reception carrier frequency assigned thereto which is determined by the transmission carrier frequency. More specifically, the receiving arrangement includes a receiving unit for receiving the repeated radio signal to produce a received radio signal of a received carrier frequency determined by the repeated carrier frequency, converting unit for converting the received radio signal in response to the reception carrier signal into a converted signal of an intermediate frequency equal to a difference between the repeated and the reception carrier frequencies, and a demodulating unit for demodulating the converted signal into a demodulated signal.
In the transmitting arrangement, it occasionally occurs that the transmission carrier frequency assigned thereto is changed for some reasons, for example, on failure of the specific transponder. In this event, the reception carrier frequency must be manually changed in the receiving arrangement.